Prunus avium is variously divided by botanists and pomologists. Whatever distinct forms of the species may exist in the wild state, they are now interminably confused by hybridization under cultivation. It is impossible to divide the species into botanical varieties from the characters of the horticultural varieties, as many botanists have attempted to do. The species can be roughly divided into two pomological groups, the distinguishing character being the texture of the flesh.
Sweet Cherries with soft, tender flesh form one group known by pomologists under the French group name Guigne or the English Gean. These are also the Heart cherries of common parlance. These soft-fruited cherries may again be divided into dark colored varieties with reddish juice and light colored sorts with colorless juice. Typical light colored Geans are Coe, Ida, Elton and Waterloo; dark colored ones are Black Tartarian, Early Purple and Eagle. It is to this group of cherries that Linnaeus gave the varietal name Juliana and De Candolle the specific name Cerasus Juliana.
The second group is distinguished by the firm, breaking flesh of the fruits—the Bigarreaus of several languages, the name originally having reference to the diverse colors of the fruits. This group is further divisible in accordance with color of fruit and juice into black Bigarreaus and light Bigarreaus. Chief of the black cherries falling into this division are Windsor, Schmidt and Mezel; of the light ones, which are much more numerous, Yellow Spanish and Napoleon are representative sorts. Linnaeus called these hard-fleshed cherries Prunus avium duracina; De Candolle called them Cerasus duracina; K. Koch, Prunus avium decumana; and Roemer, Cerasus bigarella.
Besides these two orchard forms of Prunus avium several other horticultural forms, quite as distinct or even more so, are grown as ornamentals, some of which are listed as distinct species or as botanical varieties of Prunus avium. To add to the confusion, a number of Latinized garden names are more or less commonly applied to these ornamental Sweet Cherries. Schneider,[9] in revising the genus Prunus, names four botanical forms of Prunus avium and two natural hybrids with other species.
PRUNUS AVIUM × PRUNUS CERASUS
The Duke cherries, long placed by most pomologists and botanists in a botanical variety of Prunus avium, are unquestionably hybrids between the Sweet Cherry and the Sour Cherry. A study of the characters of the varieties of the Duke cherries shows all gradations between Prunus cerasus and Prunus avium, though, in the main, they resemble the latter more than the former, differing from the Sweet Cherries most noticeably in having an acid flesh. Sterility is a common attribute of hybridism. In this respect the Dukes behave like most hybrids. In several Duke cherries all of the seeds collected at this Station are sterile; in others, most of them are sterile and in none are the seeds as fertile as in varieties known to be pure bred as to species. So, too, shrunken pollen grains indicate hybridity. A study of the pollen of the Duke cherries shows many grains, the greater proportion, to be abnormal, a condition not found in the pollen of varieties true to species. May Duke, Reine Hortense and Late Duke are the leading hybrid varieties.
PRUNUS AVIUM × PRUNUS CERASUS (REINE HORTENSE)
There are dark colored Duke cherries with reddish juice and light colored sorts with uncolored juice, just as in the two parent species. May Duke is a typical variety with colored juice while Reine Hortense is probably the best-known cherry among these hybrids with uncolored juice. About 65 of the cherries listed in The Cherries of New York are "Dukes," or hybrids between the Sweet and the Sour Cherry.
The name Duke comes from the variety May Duke which is a corruption of Médoc, a district in the department of Geronde, France, from whence this variety came. The cherries of this group are known as Dukes only in England; in France the name Royale is similarly used.